# reducing motor power?

#### Poor old sod

Joined Jul 25, 2017
190
240V 50hz 88W sewing motor, want ~50W with max torque as belt drive genset starter motor. PSU: 75W 12v/240v inverter. power resistive regulation unlikely. KISS pls.

#### Marley

Joined Apr 4, 2016
489
Sewing machine motors are generally the "universal" type - carbon brushes and commutator.
Power output only depends on the load of course.
If you want to reduce the speed, one way is to reduce to supply voltage. A better way is to use a triac regulator with tacho feedback.
For your special application, might be easier to simply change the pulley belt ratio. That way, torque increases as speed reduces.

#### jpanhalt

Joined Jan 18, 2008
11,088
Starting at Post #16 (July 13).

I elected to reference the thread rather than where it took that tangent.

#### shortbus

Joined Sep 30, 2009
9,757
240V 50hz 88W sewing motor, want ~50W with max torque as belt drive genset starter motor. PSU: 75W 12v/240v inverter. power resistive regulation unlikely. KISS pls.
Exactly how if this is a belt drive, will the "starter' disconnect when the generator starts?

#### strantor

Joined Oct 3, 2010
6,330
Exactly how if this is a belt drive, will the "starter' disconnect when the generator starts?
I don't think it needs to be disconnected. Once the engine is running (started by means other than this tiny motor) the engine would not even notice it still coupled.

Does nobody else want to acknowledge that this motor is undersized by a factor of 50+? It wouldn't start even the smallest RC model toy engines.

#### shortbus

Joined Sep 30, 2009
9,757
I don't think it needs to be disconnected. Once the engine is running (started by means other than this tiny motor) the engine would not even notice it still coupled.

Does nobody else want to acknowledge that this motor is undersized by a factor of 50+? It wouldn't start even the smallest RC model toy engines.
But the bronze bushings in the "starter motor" sure would. Think of how much faster the gearing/pulley ratio would make it turn.

I told him the sewing machine motor, or any I've ever seen on a home machine wouldn't have the power needed in the first thread he asked in. Now an induction motor like in a walking foot machine would, they're usually in a 1/3 to 1/2 HP range.

Joined Feb 20, 2016
4,251
The power limitation seems to be the 12V to 240V inverter. Get rid of that and change the motor. I think a 12V motor would be better.
Maybe even look at a battery drill?

#### jpanhalt

Joined Jan 18, 2008
11,088
Starter rings are available (about $30), if not for that engine specifically, bolt on units. I did a bit of searching earlier today. Generac may make its own engines or maybe it OEM's and rebrands them from someone else. The cost for an electric start retrofit is over$300.

Does the current flywheel have teeth?
What is the engine HP or displacement?
Assuming the flywheel doesn't have teeth, what's it diameter? Do you know anyone with a lathe?

Adding a real starter to a toothed flywheel is less than $40. The bolt-on starter ring was about$150, if you have room. Which gets back to the accurate diameter of the flywheel (behind) the fan.

#### Poor old sod

Joined Jul 25, 2017
190
genset is 750W total, 4st motor prob 2hp. will use empty starter pulley and engagement system to decouple elect motor [84w] after start. my problem is the risk of o'loading the inverter, because I think they then crowbar the output. don't want to buy another one. need to limit the current drain somehow, even if lower voltage input? inverter is ciggy socket type for car. pulley ratio is at least 10:1. a rubber belt is envisioned, M section/toothed? A single series diode is not a suitable solution. if engagement ratchet too frail, then will fit Brigg & Stratton coaster start clutch and pulley from sim. power motor.

#### jpanhalt

Joined Jan 18, 2008
11,088
750 W is relatively nothing. Why not use am aluminum knob on the flywheel/fan (like a threaded prop spinner) and a simple model airplane starting motor? Total cost <\$30. Even less, if you visit hobby sites or Craigslist for used.

I thought you were wondering about something in the 5 HP or greater range. You actually are talking about something in the power range of a 28 cc string trimmer. For up to 2.0 in^3 engine (considerably larger), those starters work great and are trouble free. You can run them off any 12 V battery. Some of the guys use lipo's or SLA garden tractor batteries in a self-contained portable unit. I just ran jumpers from the 12 V accessory socket in my car. Lamp cord suffices.

#### Poor old sod

Joined Jul 25, 2017
190
thx, that's usefull. afair it's 66cc. it all has to be self contained, and require min strength to carry/operate. genset with fuel is 8kg, my limit will be abt 10kg. Not sure of local cost of model starter unit, but will look into it. need it to be short too.

#### Poor old sod

Joined Jul 25, 2017
190
thx, that's usefull. afair it's 66cc. it all has to be self contained, and require min strength to carry/operate. genset with fuel is 8kg, my limit will be abt 10kg. Not sure of local cost of model starter unit, but will look into it. need it to be short too. model starters too weak, I'm told.
of course, I could bridge rectify and use a dc current limiter, [0.44A p max] which might be the easiest way? not quite sure how to do that tho'.

#### shortbus

Joined Sep 30, 2009
9,757
require min strength to carry/operate. genset with fuel is 8kg, my limit will be abt 10kg.
You said in the other thread the gen was hard to pull start. If you are capable of carrying and moving around 8 to 10kg, why can't you pull the starter cord? I'm doubting it would take 1/2 of that amount of weight to pull start the motor.

#### Poor old sod

Joined Jul 25, 2017
190
You said in the other thread the gen was hard to pull start. If you are capable of carrying and moving around 8 to 10kg, why can't you pull the starter cord? I'm doubting it would take 1/2 of that amount of weight to pull start the motor.
can lift dead mass, but not pull starter cord/hold genset down. without much pain. very weak. can just hold 1kg with arm extended for few sec.

#### MisterBill2

Joined Jan 23, 2018
13,742
I don't think it needs to be disconnected. Once the engine is running (started by means other than this tiny motor) the engine would not even notice it still coupled.

Does nobody else want to acknowledge that this motor is undersized by a factor of 50+? It wouldn't start even the smallest RC model toy engines.
Given the reduction ratio needed for a motor that small to turn over any engine to start a generator, the failure will happen when the engine starts and that little motor is driven way faster than it can handle. So certainly the motor would need a means to de-clutch to the engine. That should be obvious.

#### MisterBill2

Joined Jan 23, 2018
13,742
I am thinking that the desired setup is simply not possible. A sewing machine motor is not only not strong enough, in addition, it is a high speed motor and the reduction setup would be rather cumbersome and heavy. If it really is a Briggs & Stratton engine I have seen wind up starters that use a crank and a spring to spin the engine. Get one of those at a mower shop and bolt it on in place of the rope starter. Or get hold of a motorcycle starter motor setup.

#### Poor old sod

Joined Jul 25, 2017
190
can lift dead mass, but not pull starter cord/hold genset down. without much pain. very weak. can just hold 1kg with arm extended for few sec.
inverter 12v/240v 75w. =0.3A rms max. =0.42A peak. \oad 84w [0.35A rms] motor, hard start. dc resistance =
pls provide math to design current limiter with suitable 3A transistor assuming 315vp rectified dc approx sine wave. transistor assuming 315vp rectified dc approx sine wave. a serirs restor of 2.3 ohms in the emitter of a poer transistor and 2 diodws base to motor/resistor bode, and a collector base bias resistor should redulate the current/ well enough./

#### MisterBill2

Joined Jan 23, 2018
13,742
So we have a set limit of power based on the limitations of an inverter, rather than on the torque and speed required. And then we have some additional constraints on weight and size. The challenge being to spin up the 66cc gas engine , being that it is hard to start.
At full power the sewing machine motor is not likely to be able to do the job, even if it had an adequate reduction setup to provide the needed torque. I am guessing that the goal is to start the generator from a car battery connection. That would require a 12 volt motor of probably at least 200 watts.An excellent choice would be a starter motor for either a small engine motorbike, or possibly for a small garden tractor. I have seen a garden tractor that did use a combination generator/starter motor, belt driven, and actually worked. That setup was constantly engaged, since it was also the generator. That is the simplest and lightest scheme I can imagine, unless the present generator also has a DC generator built in. I actually saw one of those many years ago.